Configurable Multiplexer Loop - Texas Instruments ADS1258EVM-PDK User Manual

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3.3 Configurable Multiplexer Loop

The ADS1258 multiplexer has optional differential output connections (MUXOUTx) that enable a single signal-
conditioning circuit to be used for all analog inputs. The default signal conditioning circuit is a differential
unity-gain buffer (OPA2320) between each MUXOUTx and ADCINx pin. A low-pass filter is connected to the
output of the buffer. As shown in
resistors R25 and R35 are used to isolate the op-amp outputs from the filter capacitor.
OPA2320 is a dual op-amp
package used here in a single-
supply configuration
Only one decoupling cap is
needed at positive supply (pin
8) for both op-amps
Figure 3-3. ADS1258EVM Configurable Multiplexer Loop With a Signal-Conditioning Circuit
The signal-conditioning circuit shown in
output gain stage by changing resistors R24, R30, and R31 as per
G = 1 + 2 × (R24) / R30
Importantly, Equation 1 assumes that R31 = R24.
As an example, replacing R24 and R31 with 10-kΩ resistors and R30 with a 2.2-kΩ resistor creates a gain of
10 V/V between the MUXOUTx and ADCINx pins. Choose R24, R30, and R31 to be size 0603 resistors with a
tolerance ≤0.1% to maintain system accuracy.
Enable the multiplexer loop by setting BYPAS = 1 in the CONFIG0 register. To bypass the multiplexer loop,
set BYPAS = 0. One important caveat to the multiplexer bypass option is that the signal-conditioning circuit is
only disconnected from the ADCINx pins. In other words, the signal-conditioning circuit is still connected to the
MUXOUTx pins regardless of the state of the BYPAS bit.
3.3.1 Measuring Single-Ended Inputs Using a Pseudo-Differential Configuration
Both the buffer or gain stage configuration can be used to directly measure true differential input signals.
Measuring single-ended inputs requires additional consideration because the OPA2320 is powered by a
unipolar, 5-V supply. Additionally, the OPA2320 requires up to 45 mV of headroom between the output voltage
and either supply (V+ or V–). These two limitations restrict the ability of the default amplifier circuit shown
in Figure 3-3 to measure single-ended signals referenced to ground. Instead, the amplifier negative input
(MUXOUTN) must be level-shifted to a mid-supply voltage. This type of measurement is referred to as pseudo-
differential, or a measurement with respect to some fixed voltage above ground.
Implement a pseudo-differential measurement on the ADS1258EVM by shorting the 2.5-V REFOUT voltage to
the appropriate channel. In Auto-Scan mode, use jumper JP2 to short REFOUT to AINCOM because AINCOM
SBAU126E – MAY 2007 – REVISED JUNE 2023
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Figure 3-3, capacitor C20 is used to bypass the ADC sampling currents and
OPA2320AQDGKRQ1
U4A
MUXOUTP
3
2
R30
DNP
0
U4B
6
B
MUXOUTP
5
OPA2320AQDGKRQ1
U4C
AVDD
8
V+
OPA2320AQDGKRQ1
C17
100nF
GND
Figure 3-3 can also be configured into a differential-input, differential-
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R25
1
47.5
R24
0
R31
0
R35
7
47.5
4
V-
GND
Equation 1.
ADS1258EVM-PDK Evaluation Module
Analog Interface
ADCINP
C20
2200pF
ADCINP
(1)
7